1. Field of the Invention
The invention relates to the field of the checking of the surface quality of optical elements and its object is an imaging system with integrated measurement of the wear of its optical elements working in transmission mode, notably domes or windows for the protection of airborne optronic instruments. The invention can be applied more particularly but not exclusively to the airborne optronic instruments comprising an imaging system such as this.
An imaging system comprises a combination of optical elements, working in transmission and reflection modes, and an imaging detector. The combination of optical elements projects an image of the scene observed in the sighting direction on the imaging detector, and the imaging detector gives a video signal by which this scene can be displayed on a monitor.
In the context of the main application envisaged, it may be recalled that an optronic instrument placed on board an aircraft is constituted conventionally by an imaging system and a sighting head that is movable in relative bearing (and possibly in elevation) in order to orient the direction of the sighting line of the imaging system.
The sighting head houses a sighting line deflection mirror and has a dome or window to shield and isolate the delicate elements of the imaging system from the external atmosphere. The window and the mirror form part of the optical combination of the imaging system for they play a role in the formation of the image projected on the imaging detector.
2. Description of the related art
The spectral band of operation of the equipment determines the choice of the material constituting the window. Unfortunately, the materials (ZnSe, Ge or ZnS) selected as a function of their possibility of optical transmission in the useful ranges of infrared transmission (3-5 .mu.m and 8-12 .mu.m) do not have special qualities of hardness and of surface condition. These materials stand up poorly to the stresses generated by flight at high speeds in a hostile environment, notably to pluvial erosion, and are then exposed to the phenomenon of wear and tear.
This wear and tear depends very substantially on particular flying conditions encountered during the various missions carried out. It therefore proves to be very difficult to make a priori plans for a periodicity of replacement of the window that would be adapted to all missions.
Thus, the replacement of the window should be governed not by its duration of use but by the determining of the starting point of time at which the wear becomes such that it reduces the performance characteristics of the optronic equipment, i.e. to the point of rendering its operational use impossible.
A visual examination of the surface condition of the window most usually leads to the premature conclusion that it is worn out and hence to its replacement, thus substantially increasing maintenance costs. Besides, an examination such as this can be made only on the ground.